1. Field of the Invention
The present invention is directed to a control system for a linear vibration motor, and more particularly to a system of driving a vibrator linearly with respect to a stator at a uniform amplitude.
2. Description of the Prior Art
U.S. Pat. No. 5,632,087 discloses a linear vibration motor composed of a stator with a winding and a vibrator with a permanent magnet. A drive current is fed to the winding to generate a magnetic field with interacts with the permanent magnet to reciprocate the vibrator linearly relative to the stator. A coil sensor is additionally provided in an opposed relation with permanent magnets embedded in the vibrator to produce a voltage which is indicative of the speed of the vibrator, i.e., an ongoing amplitude of the vibrator in motion. Based upon the ongoing amplitude, a control is made to vary the drive current in order to keep the amplitude constant at a desired level. However, this control system requires the coil sensor which gives an additional cost and requires an additional space for the sensor, making it difficult to reduce the vibration motor into a compact structure with a reduced cost.
Another prior art is Japanese Patent Early Publication No. 62-285655 which discloses an actuator with a speed feedback control composed of a vibrator with first and second windings and a stator with a permanent magnet. The first windings is utilized to solely receive a drive current for moving the vibrator, while the second winding is utilized selectively to receive the drive current and to sense a voltage indicative of the speed of the vibrator for feedback control of the actuator based upon the voltage. The selection of the functions is made by a switch so that the second winding does not act to drive the vibrator while the second winding is switched to act as the sensor, and vice versa. Thus, the actuator necessitates at least two windings, which is a still hindrance to compacting the structure.
Further, Japanese Patent Early Publication No. 49-77116 discloses a voice-coil type linear motor composed of a vibrator with a moving coil and a stator with a permanent magnet. The moving coil receives a drive current to be driven thereby to vibrate and generate a voltage including a component indicative of the speed of the moving coil. An equivalent circuit to the moving coil is provided to give an output corresponding to a voltage of the moving coil in a stationary state. The output is compared to the voltage of the moving coil in motion in order to derive the component indicative of the speed of the moving coil. Bases upon thus derived component or the speed of the moving coil, a control is made to drive the moving coil. In this prior art, the equivalent circuit adds an extra bulk to the structure, making it difficult to achieve a compact design for the linear motor.
In view of the above problems, the present invention has been achieved to provide a control system for a linear vibration motor which is capable of eliminating a separate sensor of detecting the motion of the motor for reducing a number of components and a manufacturing cost as well as reducing an overall size of the liner vibration motor. The linear vibration motor comprises a stator provided with one of a permanent magnet and a winding, and a vibrator provided with the other of the permanent magnet and the winding. The winding is fed with a drive current to generate a magnetic field which interacts with the permanent magnet to cause the vibrator to reciprocate linearly relative to the stator for producing a vibration. The control system in accordance with the present invention includes a controller which detects an ongoing amplitude, i.e., the motion represented by the displacement, speed, or acceleration of the vibrator and provides ON-periods of feeding the driving current to the winding in a varying amount based upon the ongoing amplitude detected in order to keep a vibration amplitude of the vibrator constant. The controller is characterized to provide OFF-period in which the controller instructs not to feed the drive current and in which the controller detects a voltage developed across the winding and determines the ongoing amplitude based upon the detected voltage. Thus, the winding can be best utilized commonly to drive the motor and to sense the ongoing amplitude or motion of the vibrator, thereby eliminating an additional sensor for detection of the motion of the vibrator.
The vibrator vibrates at substantially a fixed frequency. Due to the know frequency, the controller provides a first OFF-period which is set to continue for a predetermined duration within which the vibrator is expected to reach a maximum displacement. Then, the controller determines a reference time within the first OFF-period when the voltage across the winding becomes zero as indicative of that the vibrator reaches the maximum displacement. Based upon the reference time, the controller determines a subsequent ON-period after a first predetermined time interval from the reference time, and determines a subsequent OFF-period after a second time interval from the reference time. Thus, the reference time can be renewed each time the voltage becomes zero in the OFF-period, enabling to provide the subsequent ON-period as well as OFF-period accurately for consistent control of the linear motor.
Preferably, the controller reads the voltage across the winding at a detection timing after a predetermined time from the reference time so as to determine the ongoing amplitude of the vibrator from the voltage.
Alternately, the controller may be designed to obtain a time difference within the OFF-period between a first timing at which the voltage reaches a first reference voltage and a second timing at which the voltage reaches a second reference voltage which is different from the first reference voltage. Then, the controller determines the ongoing amplitude of the vibrator from thus obtained time difference. In this case, the first reference voltage is set to be zero so that the controller can determine the first timing as the reference time.
Further, the controller may be designed to obtain a voltage difference between a first voltage of the winding detected at a first predetermined time from the reference time and a second voltage of the winding detected at a second predetermined time from the reference time. The first and second predetermined times are within the OFF-period and different from each other. Then, the controller determines the ongoing amplitude of the vibrator from thus obtained voltage difference.
The OFF-period may be defined within each cycle of the vibration or alternately be defined to appear only once within more than one cycle of the vibration.
The driven current is in the form of a pulse-width-modulated (PWM) signal having a variable width corresponding to the ON-period. The ON-period is limited to have a maximum width to afford the OFF-period in one cycle of the vibration.
Preferably, the controller is designed to feed the drive current partly in the form of a pulse-width-modulated (PWM) signal during a first fraction ON-period of variable width and the rest in the form of a fixed width signal during a second fraction ON-period of fixed width. The first ON-period occurs in a half cycle of a particular cycle of the vibration, while the second ON-period occurs in the other half cycle of the same cycle so that the OFF-period occurs together with the second ON-period within the other half cycle. Thus, the vibrator can receive driving forces which are generated respectively in the half cycles and act in opposite moving directions, thereby making efficiently controlled vibration.
The controller is preferred to include a low amplitude compensator which monitors the ongoing amplitude of the vibration and issues a restriction signal when the ongoing amplitude is lower than a minimum reference amplitude. In response to the restriction signal, the controller disables to provide the OFF-period and at the same time elongates the ON-period to feed the drive current to a maximum extent over the following predetermined cycles of said vibration. Thus, the vibrator can be rapidly restored to vibrate at the intended amplitude.
These and still other objects and advantageous features of the present invention will become more apparent from the following detailed description of the preferred embodiment when taken in conjunction with the attached drawings.